The process of wet oxidation has been used for the treatment of aqueous streams for over thirty years. See, for instance, U.S. Pat. No. 2,665,249 which discloses oxidation of carbonaceous dispersions in an aqueous phase.
Wet oxidation involves the addition of an oxidizing agent, typically air or oxygen, to an aqueous stream at elevated temperatures and pressures, with the resultant "combustion" of oxidizable materials directly within the aqueous phase. The wet oxidation process is characterized by operating pressures of 30 to 250 bar (440 to 3630 psia) and operating temperatures of 150.degree. to 370.degree. C. Since gas phase oxidation is quite slow at these temperatures, reaction is primarily carried out in the liquid phase. Thus, the reactor operating pressure is typically maintained at or above the saturated water vapor pressure, so that at least part of the water is present in liquid form.
Conventional wet oxidation suffers several disadvantages. First, it is limited by the degree of oxidation attainable. Second, it is unable to adequately handle refractory compounds. Third, reaction times are often too slow for industrial use. Interest therefore grew for the extension of wet oxidation to higher temperatures and pressures.
In U.S. Pat. No. 2,944,396 is described the addition of a second oxidation stage after the wet oxidation process. Unoxidized volatile combustibles accumulating in the vapor phase of the first (wet oxidation) stage are sent to a second stage operated at temperatures above the critical temperature of water of about 374.degree. C.
U.S. Pat. No. 4,292,953 discloses a modified wet oxidation process for power generation from coal and other fuels in which, as heat is liberated by combustion, the entire reaction mixture exceeds the critical temperature of water, with operating pressures of about 69 bar (1000 psi) to about 690 bar (10,000 psi) spanning both the sub- and supercritical water pressure ranges.
The wet oxidation process set forth in U.S. Pat. No. 4,338,199 has come to be known as "supercritical water oxidation" (SCWO) since oxidation is frequently conducted entirely at supercritical conditions in temperature (i.e., greater than 374.degree. C.) and pressure (greater than about 3200 psi or 220 bar). SCWO is capable of completely oxidizing virtually any organic compound in a matter of seconds at temperatures between about 500.degree. to about 600.degree. C. and pressures of about 250 bar.
A related process known as supercritical temperature water oxidation (STWO) provides similar oxidation effectiveness for certain feedstocks but at lower pressure. This process was recently described in U.S. Pat. No. 5,106,513 and utilizes temperatures in the range of 600.degree. C. and pressures between about 25 to about 220 bar. For selected feedstocks, the combination of more modest temperatures (in the range of 400.degree. to 500.degree. C.) but higher pressures (up to 1000 bar or 15,000 psi) has proven useful to keep certain inorganic materials from precipitating out of solution. See, for example, Buelow, S., "Reduction of Nitrate Salts under Hydrothermal Conditions", Proceedings of the 12th International Conference on the Properties of Water and Steam, ASME, Orlando, Fla., September, 1994.
Various processes for oxidation in an aqueous matrix will hereinafter be referred to collectively as high temperature water oxidation (HTWO) if carried out at temperatures above about 300.degree. C. and pressures of about 27.5 bar. The HTWO environment will be described below for the specific case of SCWO, though other HTWO environments will have much in common.
SCWO may be compared to incineration processes since its efficiency towards oxidizable materials is almost 100%. Indeed, much of the process development to date in SCWO has been directed toward treatment of sludges or toxic and hazardous wastes. Such materials could likewise be subjected to an incineration process.
Other potential feedstocks for the SCWO process include those wastes which are currently being handled by deep well injection techniques and wastes which have either been accumulated or spilled, including mixed radioactive/organic wastes. Due to the wide variety of potential feedstocks, the use of SCWO processes spans virtually the entire periodic table.
The products of complete oxidation in SCWO are fairly well known. Carbon and hydrogen form the conventional combustion products CO.sub.2 and H.sub.2 O. Chlorinated hydrocarbons also give rise to HCl, which will react with available cations to form chloride salts. Alkali may be intentionally added to the reactor to avoid development of high concentrations of HCl.
In contrast to normal combustion, which forms SO.sub.2, the final product of sulfur oxidation in SCWO is sulfate anion. As in the case of chloride, alkali may be intentionally added to avoid high concentrations of sulfuric acid. Similarly, the product of phosphorus oxidation is phosphate anion.
While it is frequently desirable to neutralize oxidation product anions via alkali addition, the reverse is not usually true. Feedstocks containing excess noncombustible cations are generally self-neutralized by the CO.sub.2 which evolves from oxidation. For example, a stream containing organic sodium salts will yield sodium carbonate as a product. Ammonium, another common cation, is converted to water and nitrogen (N.sub.2) or nitrous oxide (N.sub.2 O) in the SCWO process, and so does not require neutralization.
A key advantage of SCWO over incineration is the lack of NO.sub.x formation due to the relatively low temperature of operation. Oxidized forms of nitrogen, e.g., organic nitro-compounds and nitrate anion, have been found to form N.sub.2 or N.sub.2 O. When air is used as the oxidizing agent, N.sub.2 passes through the system as an inert.
While chemical equilibria under SCWO conditions has been fairly well characterized, much remains to be learned about chemical kinetics and reaction mechanisms. The situation is complicated by the wide range of densities which may exist in supercritical water systems. At typical reactor conditions of 500.degree. to 600.degree. C., the supercritical phase density is on the order of 0.1 g/cc. Reaction mechanisms are of the free radical type as with normal combustion, but are greatly affected by the higher density and water concentration which characterize SCWO conditions. On the other hand, at temperatures closer to the critical point, or in dense supercritical brine phases, densities of 0.5 to 1 g/cc are obtained and ionic reaction mechanisms dominate.
As indicated by the density employed under typical reactor conditions (0.1 g/cc), the distance between water molecules is considerably greater than the distance between molecules in normal liquid water. The disruption of hydrogen bonding causes the water molecules to lose the molecular ordering which is responsible for many of the properties of normal liquid water. In particular, solubility behavior approximates that of high pressure steam rather than that of liquid water. Smaller polar and nonpolar organic compounds, with relatively high volatility, exist as vapors at typical SCWO conditions, and hence are completely miscible with the supercritical water. Gases such as N.sub.2, O.sub.2, and CO.sub.2 show similar miscibility. Larger organic compounds, such as polymers, pyrolyze to smaller molecules at typical SCWO conditions, thereby resulting in solubilization via chemical reaction. The loss of bulk polarity of the water phase has striking effects on normally water-soluble salts, as well. No longer readily solvated by water molecules, they precipitate out as solids or dense brines. The small salt residual in the supercritical phase is largely present in molecular form, e.g., as NaCl molecules. Heavy metal oxides, having low solubility in liquid water, retain their low solubility at supercritical water conditions. Exceptions exist and high solubilities occur, however, when a metal forms a volatile salt or oxide at reactor temperatures.
The characterization of solubility behavior in the preceding paragraph has been expressed in relation to pure supercritical water. In actual SCWO systems, this behavior may be greatly altered by the presence of large quantities of gases and salts. In many applications, for example, the mass of "noncondensible" gases in the reactor may exceed the mass of water. The presence of noncondensible gases and salts in the SCWO reactor encourages the separation of phases and is similar to the "salting out" phenomenon of gases from solution.
The complexity and uniqueness of the SCWO environment, combined with the elevated temperature and pressure requirements, presents a tremendous challenge in the selection of materials of construction for commercial applications. Numerous efforts have focused on metal alloys.
While stainless steel has proven suitable for research in dealing with mixtures of water, oxygen, and hydrocarbons, commercial systems are required to handle a variety of acidic and alkaline streams, as well as streams containing a significant quantity of salts. High nickel alloys, in particular Alloys C276 and 625, have been used in testing. However, unacceptably high corrosion rates are observed with these alloys for many streams of interest. Furthermore, prolonged exposure to and cycling of these materials at reactor temperatures leads to a degradation of their mechanical properties. Both alloys are subject to embrittlement, thereby leading to the increased possibility of cracking and catastrophic failure.
A large number of metals, alloys and ceramics have been tested. Most recently, U.S. Pat. No. 5,358,645 (herein incorporated by reference) disclosed the use of zirconia based ceramics for the contact surface area of an apparatus for high temperature water oxidation of combustible materials. As stated therein, the stable form of pure zirconium oxide between ambient temperature and 1170.degree. C. is a monoclinic crystal.
As a consequence of the transformation of monoclinic zirconium oxide to its tetragonal form when heated above 1170.degree. C., ZrO.sub.2 is prone to fracturing under those thermal cycling conditions which typify high temperature processes. U.S. Pat. No. 5,358,645 discusses the use of stabilizing agents to increase the resistance of zirconium oxide to such fracturing. The most common stabilizing agents employed are yttria, magnesia, and calcia.
FIG. 1 shows how yttria alters the crystalline structure of the zirconia crystalline lattice. The basic crystalline forms of the zirconia crystal are monoclinic (M), tetragonal (T), and cubic (C). SS in FIG. 1 denotes a solid solution, i.e., the stabilizing element is integrated into the zirconia lattice. Above about 7.5 mole percent yttria, the zirconia is fully stabilized--only the cubic crystal form existing. Below about 1.5 mole % yttria, zirconia exists in its monoclinic form at room temperature. Yttria partially stabilized zirconia (YPSZ) exists at yttria mole percents between 1.5 and 7.5. At temperatures below 500.degree. C., YPSZ contains a mixture of monoclinic, cubic, and metastable tetragonal phases. The metastable tetragonal phase has been shown to be of importance in reducing the ceramic's brittleness.
A potential drawback of zirconia coatings is the possibility of the transport of oxygen through the solid phase ceramic. Degradation may result as oxygen diffuses to the metallic bond coating, thereby forming metal oxides. The formation of such oxides could render a volumetric change and lead to coating spallation. In an effort to prevent this phenomenon, it is possible to include another ceramic such as alumina as an intermediate layer. An alumina layer would be impervious to oxygen. However, alumina is unsuitable as the uppermost layer for exposure to the SCWO environment since it is considerably less inert than zirconia in many cases.
A need continues to exist therefore for a coating which demonstrates greater resistance to SCWO conditions. In particular, a need exists for a coating which may be useful for those commercial streams which are chloride-bearing.